A popular multi-band HF antenna over time has been a centre fed dipole with length around 40.23m (imperial lengths around 132′ to 136′ are often specified) with arbitrary length of arbitrary characteristic impedance two wire line.
Such a feed line operates with high standing waves, which means possibly very high voltages at some displacements along the feed line.
Is it possible to predict the terminal voltage of an installed antenna with feed line? gave a plot for terminal voltage of a specified length dipole and specified length and type of feed line.
This broad type of antenna, a centre fed dipole with two wire feed line has broadly similar characteristic of undulating terminal voltage through the useful frequency range.
This article drills down on a very popular configuration, a dipole of about 42m (132′) with 600Ω nearly low loss feed line of a length that demonstrates that under some conditions, voltage may be very high.
An NEC-4.2 model of a 40.23m dipole at 9m height above good ground, and a quarter wave of lossless 600Ω line was run to obtain a .s1p file of the impedance looking into the feed line.
Above is a plot of peak input voltage @ 1500W around the 80m band. If you are using 100W, voltage is approximately a quarter of that shown, but that is still quite substantial.
So, this was not an arbitrary length of transmission line, it was chosen to demonstrate that potentially high voltages that are incompatible with equipment can occur unless care is taken to design the antenna SYSTEM. Incompatibility includes voltage breakdown and inability to match with some ATUs.
The popular 132′ centre fed dipole with two wire feed line is not quite the no-brainer that is popularly believed.
Above is a Smith chart view of the 3.4-3.8MHz model input impedance. Changing the feed line length rotates the arc clockwise or anti-clockwise away from this high Z, high voltage zone. That seems easy enough, now repeat it all for 8 bands for a multiband antenna system.
Not surprisingly, arbitrary feed line may give arbitrary results.
Antenna SYSTEMS should be treated as a system, and designed at that level rather that a hotch-potch of components that are not well integrated.
There is nothing magic about the half wave on 80m starting point. It turns out that with a centre fed dipole with two wire line feed, it is difficult to achieve good system radiation efficiency if the dipole is less than 0.3 wavelengths, hence the popularity of antennas such as the 88′ dipole, G5RV, ZS6BKW.